Blender 2.8 Cycles Denoiser produces banding
There is banding in both your pictures. I increased the contrast so you can see clearly on any display:



You remove banding by introducing noise, the process is called dithering:

Rendered image shows visible &quot;line strokes&quot; artefacts

So the denoiser removed the noise making the banding more visible. If you'd save that in 16bit and view on 12bit display using quadro graphics there would be no visible banding.

If you'd render the image without denoiser for a lot more samples to clear the noise, you'd see the same banding problem.

If you want to have smooth looking gradients on 8bit displays you need to use dithering (introduce noise). This can be noise from Cycles itself (to be perfect Maxwell produces more natural noise for example), or if you get rid of the noise with denoiser a noise introduced in postproduction through dithering.